The present invention relates to the field of optical recording and, more particularly, to improvements in optical disk assemblies of the type in which an optically recordable and/or readable information storage medium is protected from degradation and contamination by a transparent cover sheet.
In the June 24, 1984 issue of Research Disclosure, Vol. No. 242, Item 24223, entitled, "Rigid Optical Disks", a variety of optical disk structures are disclosed for recording information in the form of tiny pits (e.g. one micron in size) in the surface of a laser write/read (LWR) layer. In the two-sided embodiment shown and described with reference to FIGS. 11-13, the recording assembly comprises a rigid, disk-shaped aluminum substrate having an LWR layer disposed on each of its opposing planar surfaces. Each of the LWR layers is protected from degradation and contamination by a thin (e.g. 75-150 microns) transparent plastic cover sheet. Each of the cover sheets is radially tensioned and spaced above its underlying LWR layer by a plastic perimeter ring which surrounds the periphery of the disk substrate, and by a smaller plastic ring concentrically arranged within the larger perimeter ring, between the LWR layer and the cover sheet. The inner ring is adhesively bonded to the disk substrate whereas the outer ring is dimensioned to have a slight interference fit with the substrate periphery. A U-shaped groove, formed in the inside wall of the perimeter ring, engages a similarly contoured chamfered surface of the substrate periphery to center the ring with respect to the substrate plane. While in the state of tension, the cover sheets are bonded to the opposing parallel surfaces of the perimeter ring, and to the radially inner plastic rings. The thickness of these rings and the tension in the cover sheets control the spacing between the LWR layer and its associated cover sheet. By virtue of the interference fit between the perimeter ring and substrate periphery, the perimeter ring has a certain degree of freedom to move in order to balance the tensioning forces exerted on the cover sheets.
As noted in the above Research Disclosure, the cover sheet/LWR layer spacing and tension are critical to the performance of the optical disk structure. Ideally, the cover sheet should be perfectly parallel to and closely spaced from its underlying LWR layer during use of the disk. Unavoidably, however, owing to the flexibility and low mass of the cover sheet, centrifugal forces acting on the air mass in the space separating the cover sheet and LWR layer produce a so-called "ballooning" effect. That is, during high speed rotation of the disk, the resulting centrifugal forces act to move the trapped air mass radially outwardly, causing the cover sheet to collapse towards the LWR layer in the central portion of the disk, and to bulge outwardly from the LWR layer at the outer portions of the disk. Physical contact between the cover sheet and LWR layer can produce catastrophic results in terms of lost data. Similarly, contact between the cover sheet and the high numerical aperture lens used to form the write/read beam on the LWR layer can damage the recording element. Thus, it is necessary to accurately control the cover sheet tension and spacing during the manufacturing process.
In the aforementioned Research Disclosure, it is proposed that the cover sheet and perimeter ring be bonded together by either ultraviolet or electron-beam-curable adhesives. While it is, indeed, possible to produce a uniform tension in the cover sheet, as well as effective seals between the cover sheet and its spacer rings, by using such adhesives, there are certain disadvantages associated with adhesives. Not only are they difficult to apply with the precision and uniformity required by high performance optical disk structures, but also they have a tendency to degrade with age, thereby causing a reduction in cover sheet tension over time. This tension reduction, of course, accentuates the aforementioned ballooning problem; furthermore, it gives rise to optical imaging problems which affect focus maintenance, tracking and data readout.